A recent development introduced the use of Ball Grid Array (BGA) technique in the manufacturing of electronic modules which can be of the Single Chip Module (SCM) type or Multi Chip Module (MCM) type. These modules are provided with a plurality of conductive pads for electrical connection with electronic circuits (such as mother boards, back planes, application boards). The electrical connection is achieved by little spherical portions of solder alloy which give the name of Ball Grid Array (BGA) to this kind of electronic module.
FIG. 1 is an example of a section of a BGA module of the SCM type. On the lower face of a substrate 101 there is a plurality of conductive pads 103 each pad provided with a solder ball 105 which will be put in contact with an electronic circuit and reflowed, thereby realizing the electrical connection. These solder balls are usually and conveniently made of an eutectic alloy containing Sn (63% by weight) and Pb (37% by weight). This alloy offers the advantage of having a relatively low melting point (183 C), known as eutectic point. On the upper face of the module there is the active element 107. The active element usually needs to be protected against atmospherical agents and against the heat of the reflow cycles. As represented in FIG. 1 the active element can be covered by a molded cap 109. Other methods of protection can be used instead, e.g. an epoxy resin underfiller.
The BGA technology has a number of advantages over the traditional technologies such as Pin Grid Arrays in terms, for example, of reliability, robustness and cost of manufacturing.
The substrates of the BGA modules can be ceramic or, more recently, organic Printed Circuit Board laminates. These last modules are usually called Plastic Ball Grid Arrays. The definition "Plastic" indicates the flexible nature of the PCB as opposed to a ceramic substrate.
During the manufacturing process it may happen that one or more of the connection balls are damaged or lost. Furthermore, once the module has been mounted on the electronic board (second level packaging), the solder balls could be proved not to provide an adequate connection between the module and the circuit board, on which the BGA module has been mounted. In such instance the BGA module must be removed from the board and it is often lost. Considering the increasing costs of the semiconductor devices, it would be obviously desirable to rework and reuse as many modules as possible once they have been found to be damaged.
The modules could be checked and fixed manually, but this activity would be far too laborious and expensive.
In order to re-introduce the reworked module in the manufacturing process, all the solder balls must be completely removed and no traces of solder material should be left on the module surface. It is known to rework ceramic packages by using hot water vapour in order to reflow the solder balls causing detachment from the substrate. However in the case of BGA package with an organic laminate the temperatures cannot be too high, because this would damage the substrate. Another problem is that the solderable surface on the module substrate could be jeopardized by the oxidation caused by the exposure to air. Furthermore the reflowing of the solder balls does not guarantee the detachment of all the solder material.
It is an object of the present invention to provide a technique which alleviates the above drawbacks of the prior art.